Magnetic resonance imaging apparatus, and control apparatus of cryogenic cooling system and control method thereof

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a magnetic resonance imaging (hereinafter, referred to as MRI) apparatus that measures a nuclear magnetic resonance signal (hereinafter, referred to as an NMR signal) from hydrogen, phosphorus or the like in an object to visualize a nuclear density distribution or a relaxation time distribution and the invention particularly relates to a technique of controlling a cryogenic cooling system in an MRI apparatus which includes a superconducting magnet.

Background Art

The MRI apparatus is an apparatus that measures an NMR signal generated by nuclear spins constituting a structure of an object, particularly a human body, and that visualizes the morphologies and functions of a head, an abdomen, four limbs, or the like of the object two-dimensionally or three-dimensionally. Since a correlation between the SN ratio of the NMR signal and the magnetic field strength of the MRI apparatus is positive, the superconducting magnet, with which a high magnetic field strength can be achieved, is used in the MRI apparatus.

The superconducting magnet is disposed in a helium container in which liquid helium is injected and is cooled to the boiling point of the liquid helium (4.2 K) so that the superconductive state of a coil, which generates a magnetic field, is maintained. Since the liquid helium evaporates and disappears, the superconducting magnet is provided with a cryogenic cooling system and has a configuration in which gasified helium is condensed and captured on a surface having a very low temperature and returns to the liquid helium. The cryogenic cooling system is configured to include a cold head that is attached to the helium container and condenses gasified helium, a cryo-cooler that cools the cold head, and a compressor that drives the cryo-cooler.

Usually, recondensation of the gasified helium is performed at all times. In addition, in a case where the pressure in the helium container excessively decreases due to the recondensation of the gasified helium, pressure control, in which a heater disposed in the helium container is caused to generate minute heat so as to gasify the helium, is performed.

As described above, the cryogenic cooling system is driven and consumes power even at a time when the MRI apparatus is not used. Therefore, for example, JP-A-11-99135 proposes a technique of suppressing power consumption in the cryogenic cooling system. In the technique of JP-A-11-99135, a timer is connected to the compressor and the compressor is halted at late night or early morning so that the cryogenic cooling system is halted. Meanwhile, JP-A-2015-54217 proposes a technique in which the cryogenic cooling system is halted during inspection using the MRI apparatus in order to prevent deterioration in image quality which is attributable to vibration of the cryo-cooler.

SUMMARY OF THE INVENTION

However, in the technique of JP-A-11-99135, the compressor and the cryogenic cooling system are continuously halted for a long period of time at late night or early morning. Accordingly, the temperature of the cold head rises and the cold head swells. Therefore, the cold head is worn when the operation of the compressor is restarted and thus there is a problem that the life span of the cold head becomes short. In addition, since the compressor is halted for a long period of time, the temperature of the helium container also rises and there is a high possibility of helium loss attributable to boil-off.

In the technique of JP-A-2015-54217, the operation of the cryogenic cooling system is controlled using a provided temperature threshold value. However, control for minimizing a change in temperature is not performed and wear of the cold head is not considered.

Meanwhile, since a load is applied to a motor when the compressor is restarted, there is a restriction on the number of times of halts of the compressor which is designated by a manufacturer. Specifically, the number of times of halts is restricted to 24 times per day and 6 times per hour, for example.

In consideration of the above-described problem, an object of the present invention is to maintain a long life span of a cold head by minimizing a change in temperature of the cold head and to reduce the amount of consumed power by shortening the operation time of a compressor with a restriction on the number of times of halts of the compressor.

In order to solve the above-described problem, the invention provides an MRI apparatus including a superconducting magnet that includes a refrigerant container and a superconducting coil, a cryogenic cooling system with which the superconducting magnet is provided, and a controller that controls the operation of the cryogenic cooling system. The cryogenic cooling system includes a cold head, a cryo-cooler, a compressor that supplies a compressed gas to the cryo-cooler, and a detection unit that detects the pressure in the refrigerant container, the refrigerant container being provided with the cold head, the cryo-cooler, the compressor, and the detection unit. An upper limit value of the maximum number of times of halts of the compressor within a predetermined period of time is set in advance. The controller intermittently halts the compressor within a range in which the pressure detected by the detection unit is equal to or lower than a predetermined upper limit value and performs control such that the number of times of halts of the compressor within the predetermined period of time reaches the upper limit value of the maximum number of times of halts.

According to the invention, it is possible to maintain a long life span of a cold head by minimizing a change in temperature of the cold head and to reduce the amount of consumed power by shortening the operation time of a compressor while satisfying a condition relating to a restriction on the number of times of halts of a compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram illustrating configurations of a superconducting magnet and a cryogenic cooling system in an MRI apparatus according to a first embodiment.

<figref idref="DRAWINGS">FIG. 2</figref> is a flowchart illustrating an operation of a digital signal processing device according to the first embodiment.

<figref idref="DRAWINGS">FIGS. 3A and 3B</figref> are graphs illustrating how the pressure in a helium container according to the first embodiment changes over time.

<figref idref="DRAWINGS">FIG. 4</figref> is a flowchart illustrating an operation of a digital signal processing device according to a second embodiment.

<figref idref="DRAWINGS">FIG. 5</figref> is a graph illustrating how the pressure in a helium container according to the second embodiment changes over time.

<figref idref="DRAWINGS">FIG. 6</figref> is a flowchart illustrating an operation of a digital signal processing device according to a third embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the invention will be described.

First Embodiment

As illustrated in <figref idref="DRAWINGS">FIG. 1</figref>, an MRI apparatus according to a first embodiment includes a superconducting magnet <b>111</b>, a cryogenic cooling system that maintains the pressure and the temperature in the superconducting magnet <b>111</b>, and a digital signal processing device (a controller) <b>105</b> that controls the operation of the cryogenic cooling system.

The superconducting magnet <b>111</b> includes a main coil <b>108</b> that generates a magnetic field, a shield coil <b>109</b> that prevents a stray magnetic field from being generated, a refrigerant (helium) container <b>107</b> that accommodates the coils <b>108</b> and <b>109</b> and cools the coils <b>108</b> and <b>109</b> to the boiling point of a refrigerant (for example, 4.2 K), a 40K shield <b>110</b> for preventing radiant heat, and a vacuum container <b>106</b> that accommodates all of these components.

The cryogenic cooling system is configured to include a cold head <b>101</b>, a cryo-cooler <b>102</b>, a compressor <b>103</b>, a heater <b>112</b>, a detection unit <b>113</b>, and a magnet monitoring unit <b>104</b>. The cold head <b>101</b> and the cryo-cooler <b>102</b> constitute a cryogenic cooling pump and an opening which is provided in the helium container <b>107</b> and the vacuum container <b>106</b> is provided with the cold head <b>101</b> and the cryo-cooler <b>102</b>. The compressor <b>103</b> supplies a compressed refrigerant gas to the cryo-cooler <b>102</b> and drives a movable unit in the cold head <b>101</b> to cool the cold head <b>101</b> to a temperature equal to or lower than the boiling point of a refrigerant. The magnet monitoring unit <b>104</b> has three functions. The first one of the functions is a function of measuring the pressure in the helium container <b>107</b> and the temperature of a wall surface of the helium container <b>107</b> from an output signal of the detection unit <b>113</b>. The second one of the functions is a function of controlling the operation and the halt of the compressor <b>103</b>. The third one of the functions is a function of operating the heater <b>112</b> in the helium container <b>107</b> to rise the temperature so that the pressure in the helium container <b>107</b> slightly rises in a case where the pressure in the helium container <b>107</b> is excessively low and is equal to or lower than a predetermined pressure.

The digital signal processing device <b>105</b> obtains the pressure and the temperature in the helium container <b>107</b> via the magnet monitoring unit <b>104</b>. In addition, the digital signal processing device <b>105</b> operates and halts the compressor <b>103</b> via the magnet monitoring unit <b>104</b> on the basis of information on the pressure and the temperature in the helium container <b>107</b>.

Accordingly, the superconducting magnet <b>111</b> generates a uniform static magnetic field in an imaging space <b>114</b>. In addition, the MRI apparatus includes a gradient magnetic field coil that applies a gradient magnetic field to the imaging space <b>114</b>, a transmission coil that irradiates an object disposed in the imaging space <b>114</b> with a high frequency pulse, a reception coil that receives an NMR signal generated by the object, and an image reconstruction unit that reconstructs an image by processing the signal received by the reception coil, in addition to the superconducting magnet <b>111</b>. The digital signal processing device <b>105</b> also can be used as the image reconstruction unit.

The digital signal processing device <b>105</b> is constituted by a processor such as a CPU and a memory and it is possible to realize at least a function of controlling the cryogenic cooling system with the processor executing a program, which is stored in the memory in advance. In addition, it is possible to realize a portion or all of functions of the digital signal processing device <b>105</b> by using a hardware such as an application specific integrated circuit (ASIC) or a field-programmable gate array (FPGA).

An operation in which the digital signal processing device <b>105</b> controls the cryogenic cooling system will be described below with reference to <figref idref="DRAWINGS">FIG. 2</figref>. In the following description, “pressure” and “temperature” mean the pressure and the temperature in the helium container <b>107</b>.

In the first embodiment, the digital signal processing device <b>105</b> intermittently halts the compressor <b>103</b> within a range in which the pressure detected by the detection unit <b>113</b> is equal to or lower than a predetermined upper limit value and performs control such that the number of times of halts of the compressor <b>103</b> within the predetermined period of time reaches the upper limit value of the maximum number of times of halts of the compressor <b>103</b>. For example, as illustrated in <figref idref="DRAWINGS">FIG. 3A</figref>, the digital signal processing device <b>105</b> operates the compressor <b>103</b> in a case where an elapsed time after the compressor <b>103</b> is halted reaches a predetermined “halt time” or the pressure detected by the detection unit <b>113</b> reaches a predetermined upper limit pressure value (Upper limit) and the digital signal processing device <b>105</b> halts the compressor <b>103</b> again in a case where the pressure reaches a predetermined lower limit pressure value (Lower limit) and a predetermined “cycle time” has elapsed after a previous halt. Whenever the cycle time elapses, the digital signal processing device <b>105</b> calculates the cycle time and the halt time to halt the compressor <b>103</b> a number of times equal to the number of remaining times of halts within the remainder of the predetermined period of time and resets the cycle time and the halt time. Hereinafter, this will be described in detail below.

First, in Process <b>201</b> in <figref idref="DRAWINGS">FIG. 2</figref>, the digital signal processing device <b>105</b> sets an operation halt ratio HaltRatio [unitless] and a pressure rise rate Vp [Pa/s].

The operation halt ratio HaltRatio is a ratio of the halt time of the compressor <b>103</b> to a time between a previous halt of the compressor and a current halt of the compressor (the cycle time). As the “cycle time” with which an initial value of the operation halt ratio set in Process <b>201</b> is determined, a time required to return the pressure, which has risen due to a halt of the compressor <b>103</b> for the halt time, to the pressure before the halt of the compressor is used. The operation halt ratio HaltRatio is influenced by the atmospheric pressure, external heat input, radiant heat attributable to a gradient magnetic field of the MRI apparatus, and the performance of the compressor <b>103</b> and the cryo-cooler. However, here, the operation halt ratio HaltRatio is set to a predetermined value while not considering individual differences between magnets and the environment. For example, the initial value of the operation halt ratio HaltRatio is a value of ⅓ or the like. This means that a time required to return the pressure to the pressure before the halt of the compressor (the cycle time) is approximately two times the halt time of the compressor.

The pressure rise rate Vp [Pa/s] is a rate at which the temperature rises for the halt time of the compressor <b>103</b>. This parameter is influenced by external heat input and radiant heat attributable to a gradient magnetic field of the MRI apparatus. However, here, the pressure rise rate is set to a predetermined value. The pressure rise rate is set to a value of 0.38 [Pa/s] or the like on the basis of experiences of the inventors.

In Process <b>202</b>, the digital signal processing device <b>105</b> operates the compressor until the pressure P [Pa] reaches a predetermined lower limit pressure value LowerLimit [Pa].

When the pressure reaches the lower limit pressure value, in Process <b>203</b>, the digital signal processing device <b>105</b> sets the maximum number of times of halts N [1/day] of the compressor <b>103</b>. As an upper limit value of the maximum number of times of halts N [1/day] within a predetermined period of time (for example, one day) (hereinafter, referred to as the maximum number of times of halts), a value designated by a manufacturer of the compressor <b>103</b> is used. In the first embodiment, a unit time for controlling the cryogenic cooling system is set to 24 hours (one day) and thus a unit of the maximum number of times of halts N is [1/day]. The maximum number of times of halts N is a value of 24 [1/day] or the like.

In Process <b>204</b>, the digital signal processing device <b>105</b> calculates initial values of the cycle time Tc [s] and the halt time Th [s] using the operation halt ratio HaltRatio and the maximum number of times of halts N according to following Equations (1) and (2), respectively. <br /><i>Tc</i>=(24×60×60)/<i>N</i>  (1)<br /><i>Th=Tc</i>×HaltRatio  (2)

According to Equation (1), the initial value of the cycle time Tc [s] is set to a value which is obtained by equally dividing 24 hours by the maximum number of times of halts N of the compressor [1/day]. According to Equation (2), the initial value of the halt time Th [s] is set to the halt time of the compressor <b>103</b> within the cycle time Tc. Hereinabove, processes before the first halt of the compressor have been described.

Subsequently, a current time is recorded in Process <b>205</b> and the digital signal processing device <b>105</b> causes the compressor <b>103</b> to enter a halted state in Process <b>206</b>.

In Process <b>207</b>, the digital signal processing device <b>105</b> waits for an arbitrary time (for example, 60 seconds). The waiting time may be changed depending on the state of the MRI apparatus. For example, the waiting time during an imaging operation which is performed by the MRI apparatus may be set to be short (for example, 10 seconds) since it is expected that there is a large change in pressure during the imaging operation.

In Branch <b>208</b>, the digital signal processing device <b>105</b> determines whether the current pressure P detected by the detection unit <b>113</b> has reached the predetermined upper limit pressure value UpperLimit. In a case where the pressure P is greater than UpperLimit, the digital signal processing device <b>105</b> proceeds to Process <b>210</b> and operates the compressor <b>103</b>. On the other hand, in a case where the pressure P is equal to or lower than UpperLimit, the digital signal processing device <b>105</b> proceeds to Branch <b>209</b>. Here, the upper limit pressure value UpperLimit is a predicted value of the maximum pressure within the cycle time and is calculated according to following Equation (3) by using the pressure rise rate Vp of which the initial value is set in Process <b>201</b> and the halt time Th obtained in Process <b>204</b>. <br />UpperLimit=<i>Vp×Th</i>  (3)

As described above, the digital signal processing device <b>105</b> operates the compressor <b>103</b> in a case where the pressure P has reached the upper limit pressure value UpperLimit so that a sharp change in pressure, which is attributable to application of the gradient magnetic field or the like, can be prevented.

Meanwhile, in Branch <b>209</b>, the digital signal processing device <b>105</b> confirms whether the halt time Th which is set in Process <b>204</b> has elapsed or not. In a case where an elapsed time after the halt of the compressor <b>103</b> is shorter than the halt time Th, the digital signal processing device <b>105</b> returns to Process <b>207</b> again and waits for another predetermined time. In a case where an elapsed time after the halt of the compressor <b>103</b> is equal to or longer than the halt time Th, the digital signal processing device <b>105</b> proceeds to Process <b>210</b> and the digital signal processing device <b>105</b> switches the state of the compressor to an operated state in Process <b>210</b>.

In Process <b>211</b>, the digital signal processing device <b>105</b> waits until the waiting time elapses as with Process <b>207</b> and proceeds to Branch <b>212</b> thereafter.

In Branch <b>212</b>, the digital signal processing device <b>105</b> proceeds to Process <b>214</b> in a case where the sum of the halt time of the compressor and the operation time of the compressor is longer than the cycle time Tc and the pressure P has reached the lower limit pressure value LowerLimit. Otherwise, the digital signal processing device <b>105</b> returns to Process <b>211</b>. In addition, in Branch <b>212</b>, when the pressure P reaches the lower limit pressure value LowerLimit for the first time after the compressor is operated, a time between the start of the operation of the compressor and the current time is recorded as CooldownTime.

In Process <b>214</b>, the digital signal processing device <b>105</b> recalculates the operation halt ratio HaltRatio and the pressure rise rate Vp [Pa/s] according to following Equations (4) and (5) by using the actual net halt time HaltedTime [s] of the compressor <b>103</b> in the current cycle time Tc and the maximum pressure MaxPressure [Pa] and updates operation halt ratio HaltRatio and the pressure rise rate Vp [Pa/s]. <br />HaltRatio=HaltedTime/(HaltedTime+CooldownTime)  (4)<br /><i>Vp</i>=MaxPressure/HaltedTime  (5)

Next, in Process <b>215</b>, the digital signal processing device <b>105</b> calculates the maximum number of times of halts N of the compressor <b>103</b> within the remainder of the predetermined period of time (24 hours) according to Equation (6) and updates the maximum number of times of halts N. That is, the maximum number of times of halts N updated according to Equation (6) indicates how many times the compressor <b>103</b> is halted within the remainder of the predetermined period of time (24 hours). <br /><i>N=N−</i>1  (6)

In Process <b>216</b>, the digital signal processing device <b>105</b> recalculates the cycle time Tc [s] and the halt time Th [s] according to Equations (7) and (8) by using the updated operation halt ratio HaltRatio and the updated maximum number of times of halts N. <br /><i>Tc</i>=(24×60×60−ElapsedTime)/<i>N</i>  (7)<br /><i>Th=Tc</i>×HaltRatio  (8)

Here, ElapsedTime [s] is the elapsed time [s] from a time of a previous halt of the compressor which is recorded in Process <b>205</b>. The right-hand side of Equation (7) represents a time which is obtained by equally dividing a period of time, which is obtained by subtracting ElapsedTime [s] from the predetermined period of time (24 hours), by the maximum number of times of halts N updated in Process <b>215</b>.

Then, in Branch <b>217</b>, in a case where the elapsed time after the time recorded in Process <b>205</b> is not longer than the predetermined period of time (24 hours), the digital signal processing device <b>105</b> returns to Process <b>206</b> and executes the next cycle. In a case where the elapsed time is longer than the predetermined period of time (24 hours), the digital signal processing device <b>105</b> returns to Process <b>203</b> and starts an operation for a new predetermined period of time.

<figref idref="DRAWINGS">FIGS. 3A and 3B</figref> illustrate a change in pressure in the helium container <b>107</b>. A graph <b>301</b> in <figref idref="DRAWINGS">FIG. 3A</figref> illustrates a change in pressure pertaining to a case where the cycle time Tc is long and the halt time of the compressor <b>103</b> is long and a graph <b>302</b> in <figref idref="DRAWINGS">FIG. 3B</figref> illustrates a change in pressure pertaining to a case where the cycle time Tc is short and the halt time of the compressor <b>103</b> is short. The cycle time of the graph <b>302</b> is short in comparison with the graph <b>301</b> and thus the change in pressure is small. Accordingly, a change in temperature of the cold head is also small in comparison with a case of the graph <b>301</b>. When a control process of the cryogenic cooling system which has been described with reference to the flowchart in <figref idref="DRAWINGS">FIG. 2</figref> is performed, since the compressor <b>103</b> is halted a number of times equal to the upper limit value of the maximum number of times of halts within the predetermined period of time, it is possible to shorten the cycle time as much as possible and the change in pressure becomes small as illustrated by the graph <b>302</b>. Therefore, it is possible to reduce the amount of consumed power by halting the operation while preventing the life span of the cold head from being shortened due to the operation being halted.

As described above, in the related art, a restriction on the number of times of halts of the compressor is not considered. However, in the first embodiment, the halt time and the operation time of the compressor <b>103</b> are determined such that the compressor <b>103</b> is halted a number of times equal to the upper limit of the maximum number of times of halts with the restriction on the number of times of halts of the compressor. Therefore, according to the first embodiment, it is possible to shorten the continuous halt time of the compressor <b>103</b> while suppressing the amount of consumed power and thus a change in temperature of the cold head <b>101</b> is minimized and wear of the cold head <b>101</b> is suppressed. In addition, according to the first embodiment, since the cycle time is short and each halt time of the compressor <b>103</b> is also short, a pressure rise magnitude is small and a possibility of boil-off can be lowered.

Second Embodiment

In a second embodiment, the cryogenic cooling system is controlled such that the compressor is continuously operated in a typical time period in which the MRI apparatus is used and a certain level of pressure rise is allowed for the other time periods.

Specifically, in the second embodiment, the digital signal processing device <b>105</b> sets a continuous operation time, which is set in advance, in the predetermined period of time. In addition, the digital signal processing device <b>105</b> calculates the cycle time and the halt time to halt the compressor <b>103</b> a number of times equal to the number of remaining times of halts within the remainder of the predetermined period of time which is obtained by subtracting the continuous operation time from the predetermined period of time and resets the cycle time and the halt time. At this time, the digital signal processing device <b>105</b> allows that the lower limit pressure value is reset to be higher than a previous value whenever the cycle time elapses. The other configurations of the second embodiment are the same as those in the first embodiment.

A flow of a process in the digital signal processing device <b>105</b> of the second embodiment will be described in detail with reference to <figref idref="DRAWINGS">FIG. 4</figref>. Processes in <figref idref="DRAWINGS">FIG. 4</figref> which are the same as the processes in <figref idref="DRAWINGS">FIG. 2</figref> of the first embodiment are given the same reference numerals.

Hereinafter, processes in <figref idref="DRAWINGS">FIG. 4</figref> will be described while focusing on a difference from the first embodiment. First, in Process <b>401</b>, the digital signal processing device <b>105</b> sets the continuous operation time in addition to the operation halt ratio HaltRatio [unitless] and the pressure rise rate Vp [Pa/s] as illustrated in <figref idref="DRAWINGS">FIG. 5</figref>. The continuous operation time indicates a time period in which the compressor is continuously operated and the continuous operation time is preferably set to a time period in one day in which inspection using the MRI apparatus is performed. For example, the continuous operation time is designated to 9:00 to 17:00. Since the helium container <b>107</b> can be sufficiently cooled within the continuous operation time, in the second embodiment, the compressor <b>103</b> is intermittently halted a number of times equal to the upper limit number of the maximum number of times of halts in time periods other than the continuous operation time. Therefore, in the time periods other than the continuous operation time, the pressure at the end of the cycle time is allowed to be higher than the pressure at the start of the cycle time (that is, the end of the previous cycle time). In addition, the operation halt ratio HaltRatio is also set to be higher than that in the first embodiment in consideration of the continuous operation time. Specifically, the operation halt ratio HaltRatio is set according to following Equation (9).

<maths id="MATH-US-00001" num="00001"><math overflow="scroll"><mtable><mtr><mtd><mrow><mi>HaltRatio</mi><mo>=</mo><mfrac><mrow><mn>24</mn><mo>×</mo><mi>OriginalHaltRatio</mi></mrow><mrow><mn>24</mn><mo>-</mo><mi>ContinuousRunning</mi></mrow></mfrac></mrow></mtd><mtd><mrow><mo>(</mo><mn>9</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>

Here, OriginalHaltRatio is the operation halt ratio required to return the pressure to the original pressure within the cycle as with HaltRatio in Process <b>201</b> of the first embodiment. In addition, ContinuousRunning is the continuous operation time and is 8 [h] in a case where the continuous operation time is 9:00 to 17:00 as described above.

Processes <b>202</b> and <b>203</b> are performed in the same manner as in the first embodiment. In Process <b>402</b>, the same calculation method as in Process <b>204</b> of the first embodiment is used except that HaltRatio obtained according to Equation (9) is used. In addition, in Process <b>209</b>, in a case where the halt time has elapsed, the digital signal processing device <b>105</b> proceeds to Branch <b>404</b> and the digital signal processing device <b>105</b> determines whether the current time is within the continuous operation time (9:00 to 17:00). In a case where the current time is within the continuous operation time, the digital signal processing device <b>105</b> proceeds to Process <b>406</b>. In Process <b>406</b>, the digital signal processing device <b>105</b> operates the compressor <b>103</b> for a predetermined time. Process <b>406</b> is terminated under a condition that the continuous operation time of the compressor has elapsed and the pressure is equal to or lower than above-described LowerLimit.

On the other hand, when the current time is not within the continuous operation time, the digital signal processing device <b>105</b> proceeds to Branch <b>403</b> after Process <b>211</b> and determines whether or not the cycle time has elapsed and the pressure is equal to or lower than TempLimit. Here, TempLimit is defined according to following Equation (10). <br />TempLimit=InitialPressure+RiseRatio×<i>t</i>  (10)

Here, InitialPressure is the pressure at the start of the cycle. That is, InitialPressure is the pressure right before the compressor <b>103</b> is halted in Process <b>206</b>. In addition, RiseRatio is the predetermined allowed pressure rise rate per unit time. For example, if a rise in pressure of 50 [Pa] per hour is allowed, RiseRatio is approximately 0.0139 [Pa/s]. Furthermore, t in Equation (10) is the elapsed time [s] in the cycle. In a case where the pressure is decreased to a pressure which is higher than the pressure at the start of the cycle by RiseRatio×t and which is calculated according to Equation (10) and the cycle time has elapsed, the digital signal processing device <b>105</b> returns to Process <b>206</b> after the operation halt ratio or the like is updated in Processes <b>214</b> to <b>216</b>, halts the compressor <b>103</b>, and proceeds to the next cycle.

In Process <b>403</b>, in a case where the cycle time has not elapsed or the pressure is not equal to or lower than TempLimit, the digital signal processing device <b>105</b> proceeds to Branch <b>405</b>. In Branch <b>405</b>, in a case where the current time is within the continuous operation time, the digital signal processing device <b>105</b> proceeds to Process <b>406</b> and operates the compressor for a predetermined time.

<figref idref="DRAWINGS">FIG. 5</figref> illustrates a change in pressure pertaining to a case where the second embodiment is applied. The pressure is decreased to LowerLimit in the continuous operation time while the pressure rises each time the cycle is repeated in time periods other than the continuous operation time. In addition, in time periods other than the continuous operation time, the operation is halted a number of times equal to the upper limit value of the maximum number of times of halts of the compressor <b>103</b>.

As described above, in the second embodiment, even in a case where a time in which the compressor is continuously operated is provided, the operation is halted a number of times equal to the upper limit value of the maximum number of times of halts of the compressor <b>103</b>. Therefore, it is possible to reduce the amount of consumed power while suppressing a change in pressure. In addition, it is possible to reduce a possibility of boil-off which is caused by a sharp change in pressure during use of the MRI apparatus since the time in which the compressor is continuously operated is provided. In addition, since a certain level of rise in pressure during a non-continuous operation time is allowed in consideration of a fact that the helium container is sufficiently cooled in the continuous operation time so that the pressure is decreased, it is possible to lengthen the halt time of the compressor.

Third Embodiment

In a third embodiment, the compressor <b>103</b> is halted for each halt time as with the first embodiment and the compressor is continuously operated after an accumulation of halt times reaches a predetermined time. The other configurations are the same as those in the first embodiment.

A flow of a process in the third embodiment is illustrated in <figref idref="DRAWINGS">FIG. 6</figref>. Processes in <figref idref="DRAWINGS">FIG. 6</figref> which are the same as those in the first embodiment are given the same reference numerals. Hereinafter, processes which are different from those in the first embodiment will be described. First, the digital signal processing device <b>105</b> proceeds to Process <b>601</b> and calculates an accumulated halt time Ts so far after Processes <b>201</b> to <b>216</b> are performed in the same manner as in the first embodiment. The accumulated halt time Ts is the total time in which the compressor <b>103</b> is halted within the predetermined period of time (24 hours). In Branch <b>602</b> which is subsequent to Process <b>601</b>, the digital signal processing device <b>105</b> compares the accumulated halt time Ts with a regulation halt time Td and continuously operates the compressor <b>103</b> for the remainder of the predetermined period of time in a case where the accumulated halt time Ts is longer than the regulation halt time Td. That is, a time in which the compressor is continuously operated is 24 hours after the time recorded in Process <b>205</b>.

The regulation halt time Td is an upper limit value of a time in which the compressor is halted within 24 hours and is determined in advance on the basis of a decrease in amount of consumed power caused by a halt of the compressor. For example, in a case where the compressor consumes power of 5 kW, the regulation halt time Td is set to 4 [h] in a case where it is desired that the compressor consumes power of 20 kWh per day.

In a case where the accumulated halt time Ts is not longer than the regulation halt time Td in Process <b>602</b> or after the compressor <b>103</b> is continuously operated in Process <b>603</b>, the digital signal processing device <b>105</b> proceeds to Branch <b>217</b>. The other processes are performed in the same manner as in the first embodiment.

As described above, in the third embodiment, the regulation halt time Td is defined and the compressor is continuously operated without being halted in a case where the halt time of the compressor is longer than Td. Accordingly, it is possible to minimize wear of the compressor and the cold head while minimizing the number of times of changes in temperature accompanying a halt of the compressor.

Note that, the invention is not limited to the above-described embodiments. In the above-described embodiments, timing of a halt of the compressor is determined on the basis of the pressure in the helium container. However, the determination may be performed on the basis of the temperature in the helium container. In addition, in the above-described embodiments, an example in which a unit time for controlling the cryogenic cooling system is set to 24 hours has been described. However, the unit time may not be 24 hours. In addition, the determination in Branch <b>208</b> may be changed to determination on whether or not an imaging operation of the MRI apparatus is started or determination on whether or not there is a change in pressure which is greater than the cooling capacity of the cryogenic cooling system. Furthermore, in the second embodiment, a time period in which the MRI apparatus is operated is set. In addition to this, the MRI apparatus may be set not to be operated at weekends or a holiday.